The advance into former insurgent strongholds that had largely been calm before the Americans withdrew less than three years ago is spreading fear that Prime Minister Nouri al-Maliki, struggling to hold onto power after indecisive elections, will be unable to stop the Islamic militants as they press closer to Baghdad.

Fighters from the Islamic State of Iraq and the Levant militant group took control Tuesday of much of Iraq's second-largest city, Mosul, sending an estimated half a million people fleeing from their homes. As in Tikrit, the Sunni militants were able to move in after police and military forces melted away after relatively brief clashes.

The group, which has seized wide swaths of territory, aims to create an Islamic emirate spanning both sides of the Iraq-Syria border.

The capture of Mosul — along with the fall of Tikrit and the militants' earlier seizure of the western city of Fallujah — have undone hard-fought gains against insurgents in the years following the 2003 invasion by U.S.-led forces.

The White House said the security situation has deteriorated over the past 24 hours and that the United States was "deeply concerned" about ISIL's continued aggression.

There were no reliable estimates of casualties or the number of insurgents involved, though several hundred gunmen were in Tikrit and more were fighting on the outskirts, said Mizhar Fleih, the deputy head of the municipal council of nearby Samarra. An even larger number of militants likely would have been needed to secure Mosul, a much bigger city.

Buck Sexton, a former CIA officer, and veteran Pete Hegseth were on “The Kelly File” tonight to weigh in.

“We’re watching one of the single greatest failures in American foreign policy unfold before our eyes,” Hegseth said, adding that President Barack Obama “is obsessed with ending wars" regardless of whether or not we win them or finish them properly.

“President Obama doesn’t care about what’s happening in Iraq right now,” he said.